Mr Nicholls, author of One Day

You would imagine that Mr Nicholls, the author of a book that has sold more than a million copies in the UK alone, been translated into 37 languages, and now been made into a major film, might have a bit of swagger to his step. Instead, the 44-year-old author of One Day is hunched on the edge of an overstuffed brown leather sofa in a suite at New York's sumptuous Soho Grand Hotel, cowed with schoolboy-ish awe.

"There's a view of the Empire State building from my window!" he exclaims. "I'm such a sucker for all this - I mean, they tell me at reception that I've got some money to spend in the hotel and I'm like this child - you'd think I'd never seen or tasted orange juice before." The orange juice - and the rest - are thanks to the people who have transferred One Day to celluloid. The story follows Dexter, played by Mr Jim Sturgess, and Emma, played by Ms Anne Hathaway, from the day that they graduate from the same university, every year for One Day for the next 20 years. Sounds simple enough, and yet no other book has such a reputation for reducing grown men and women to tears on public transport.

"I love the messages I get," Mr Nicholls smiles, "you know, 'you made me cry on the number 73 bus' - that's great. I wanted it to be a big, emotional book and tried to not be too embarrassed about that."

Perhaps the most extraordinary thing about the book, is how much men love it. It wasn't intentional though. "If you have this notion of the traditional man or woman in your head when you write, you start to pander to stereotypes," he says. "A lot of contemporary so-called romantic fiction or film tends to put men and women in diametrically opposed camps and in reality we have much more in common. Our lives are constantly thrown into disruption and chaos and passion by the people we fall in love with, and that's as true for men as it is for women. So why shouldn't a man relate to a love story as much as a woman?" Nonetheless, he's been surprised by the volume and tenor of letters he has received from men.

Our lives are constantly thrown into chaos by the people we fall in love with. So why shouldn't a man relate to a love story as much as a woman?

"I think it's getting passed around in Afghanistan!" he laughs. "I've had a number from soldiers - really quite hard-bitten male readers - who've had it forced upon them. I mean, I love Emma, but Dexter's the one who has the journey - he's a man who in many ways is awful, but who is redeemed through his friendship with this terrific, smart, funny woman. That's true for a lot of my male friends - without their female partners they really would lead disastrous lives." And is it true for him?

"Oh sure. I definitely think of my life in terms of 'before and after'. Everything before the age of 31 when I met my partner is a bit of a blur, not because I was leading a sort of Dexter-style decadent lifestyle - I was much more sheepish and stay-at-home - but life definitely got better."

Interestingly, Mr Nicholls often charts the moral decline of his novels' male protagonists with their penchant for high-end labels, something particularly true for Dexter, whose moral void is echoed in a synthetic black Prada shirt that aggravates his comedown sweats. The shirt seems to symbolise all that is wrong with his life. Mr Nicholls laughs good-naturedly and agrees.

Mr Jim Sturgess and Ms Anne Hathaway play the main characters, Dexter and Emma, in a scene from the film adaptation of One Day

"My worry is the book is perhaps a little bit puritanical and judgmental. And that going to swanky bars and wearing expensive clothes is a sort of symbol of moral decline, which of course they needn't be in normal life."

As a counter, he points out that Emma becomes much more stylish as the story goes on, and in her that's "a sign of confidence and self-belief".

"But," he continues, "I think it was just an observation really, of how a lot of my friends responded to success in their twenties and to the atmosphere of Britain in the 1990s. In the 1980s you often felt a little sheepish about traditional male enthusiasms and there was definitely a puritanical atmosphere. Within the space of a very few years that all seemed to evaporate and we went from drinking pints of bitter to all knowing about Martinis. I suppose I always felt a little absurd in those places."

When it comes to personal style, Mr Nicholls maintains he still feels self-conscious about spending money on clothes although he is, it must be said, looking quietly stylish in a crisp mid-blue shirt from the niche London label Folk.

I worry the book is a bit judgmental. And that going to swanky bars and wearing expensive clothes is a symbol of moral decline, which of course they needn't be

"I've definitely been obliged to try a little bit harder," he concedes, plucking awkwardly at his attire. "When I met Hannah she threw away all my clothes. A lot of cardigans, a lot of suede desert boots. My white socks bit the dust. And any sort of top with a zip. She has very good taste and is much more stylish than I am, so I defer to her judgment." Head bowed, he emits a small chuckle: "If I buy clothes now without Hannah around I feel a little bit as if I've done something naughty."

Mr Nicholls graduated from Bristol University in 1988 with a degree in drama and English and embarked on a career as an actor. He protests that he was terrible ("hammy and over-enthusiastic, puppyish and eager and completely graceless - clumsy and awkward and mannered") but nonetheless worked steadily for most of his twenties.

"If you have even a small tendency to envy and bitterness then acting will really bring it out. When my contemporaries started to have a lot of success in their twenties and buy their own flats and swank about in designer threads, the jealousy started to outweigh the pleasure." He swiftly realised writing was where his talent lay. Not that he's renounced acting entirely: look closely and you'll spot a cameo in One Day, which, I apologetically tell him, I missed.

"Everyone will miss it, that's deliberate!" he laughs. "I walk up some stairs, that's all. I didn't realise until I saw the film just how far away the camera was - I don't think Lone [Scherfig, the director] was taking any chances."

Watching the film was, he says, "one of the most nerve-wracking things I've done". And, satisfyingly, it also made him cry. It was the scene in which Dexter and his father, both grief-ravaged, watch TV together that did it for him. "Still, I wasn't as bad as the people beside me - for about 45 minutes they were in pieces," he says, adding with a little wry grin, "completely overreacting".